The endogenous dpp locus has been modified so that it contains the following sequence downstream of the promoter: 1. an artificial FRT cassette containing dpp tagged with Tag:HA, and 2. two tandem copies of GFP (joined by a flexible linker) that are tagged at the N-terminal end with the Tag:SS(wg) signal peptide. This results in a conditional allele: in the absence of FLP, Tag:HA-tagged dpp is expressed. Upon excision of the FRT cassette, dpp function is removed, and the secreted GFP dimer is expressed instead.